Specific gravity, in the case of an aqueous condition, is the ratio of the weight of any volume of the solution to the weight of an equal volume of water taken as the standard, the measurement being made at the same temperature or, as is sometimes the case, at a stated reference temperature. Accordingly, the specific gravity denotes the ratio of the density of the solution with respect to water for the aqueous environment.
There are many important applications in different industrial fields which require the determination of the specific gravity of a liquid such as water softening and deionization, paper and pulp, tanneries, brewing, food processing, mining, agriculture, etc. Although the present invention can be used in a broad range of disciplines, for simplicity the background of the invention is discussed in relation to the determination of the specific gravity of urine, which is a function of several parameters including the number, valence force, and weight of each dissolved solute in the sample.
Prior to the present invention, numerous physical methods have been utilized for directly measuring the specific gravity of urine. Some of the most reliable ways include the use of hydrometers, urinometers, pycnometers, dilatometers, gravimeters, and refractometers. However, most of these instruments utilize cumbersome procedures and require calibration before actually being applied to the measurement of specific gravity in the aqueous environment.
An alternative method and device to indirectly determine the specific gravity by measuring the osmolality of urine was disclosed in U.S. Pat. No. 4,015,462. The method utilizes a test strip incorporated with osmotically permeable microcapsules, which contain a solute and a coloring substance. When the strip is brought into contact with the test solution having a lower osmolality or specific gravity than that of the solute inside the microcapsules, the increase of the hydrostatic pressure in the capsules leads to its rupture and releasing of colored dye contents. The intensity of the color formed is proportional to the specific gravity of the test solution. However, the accuracy of specific gravity measured by the osmolality approach could be affected by non-ionic species which are abundantly present in the sample solution.
Another method, described in U.S. Pat. No. 4,108,727, which utilizes an ionizing agent capable of converting the non-ionic solute to ionizing species of the test solution, is an improvement to eliminate the potential source of inaccuracy caused by the nonionic species in the test sample. However, the microcapsule method on the test strips is difficult to perform for production purpose.
A further method of estimating the specific gravity or the ionic strength without measuring it directly is discussed in U.S. Pat. No. 4,318,709. The test sample is mixed with a reagent composition comprising weakly acidic or basic polyelectrolyte polymers which have been partially neutralized to an extent of at least 50% with a base (such as sodium hydroxide ) or an acid (such as hydrochloric acid), and a pH-sensitive compound.
The ionic strength of the solution can be determined by the degree of intramolecular pH alteration in the polymer and the deepness of the color being developed by a pH indicator, as there exists ion exchange between the polyelectrolyte polymer and the test sample. The specific gravity can therefore be subsequently decided by the principle of proportionality between the specific gravity and the ionic strength of an aqueous environment.
Similarly, U.S. Pat. No. 4,532,216 discloses another method which comprises a weakly acidic polyelctrolyte polymer, in which at least 50 percent of the carboxyl groups of which are present in the form of a quaternary ammonium salt, and a pH indicator.
Polyelectrolyte polymers are designated as long-chained (sometimes branched) organic polymers with a multiplicity of ionizable functional groups. When they are placed in water, the type of charge and degree of ionization of these functional groups will determine the charge character of the polyelectrolyte polymer. The weak polyanionic and weak polycationic polymers, as well as the strong polyanionic and strong polycationic polymers are polyacrylic acid and poly(vinylamine), and poly(vinylsulfuric acid) and poly(vinylbenzylammonium chloride) respectively, being mentioned by way of examples.
U.S. Pat. No. 4,376,827 and European Pat. Application No. 023,631 describe a reagent composition for determining the ionic strength or specific gravity of an aqueous test sample. Besides a strongly acidic or strongly basic polyelectrolyte polymer, it contains a buffer substance capable of providing a pH of at least 5.5 and 6.5, respectively, for each patent, as well as a pH indicator.
The disadvantage of using polyelectrolyte polymers in a reagent composition is that the light scattering which is characteristic of solutions of polyelectrolyte macromolecules may become sufficiently pronounced by lowering the charge density on the polymer after simple electrolytes specifically bind to the macroions in the test solution. Thus, the color being developed by the pH chromogenic compound in response to changes in the pH of an aqueous system may shift over time. The result of this slight color change can lead to erroneous interpretations of the specific gravity or ionic strength being measured from the test strips visually or by an instrument.
U.S. Pat. No. 5,064,615 discloses a reagent composition which does not contain any polyelectrolyte polymer for measuring the ionic strength or the specific gravity of aqueous liquids. Besides a pH indicator, the test reagent comprises at least one pH buffer substance, or at least one pH buffer substance and/or at least one complex former. The complex formers include crown ethers, cryptands, podants and multidentate ligands which contain weakly acidic or weakly basic functional groups.
The properties of the metal ions in the urine and the multidentate ligands in the reagent composition determine the extent of complex formation. The electrostatic forces between small ions of high charge and the said ligands are particularly strong and lead to stable complex forners. However, the univalent large alkali metal ions in the urine such as potassium and rubidium mostly exist as hydrated ions and probably do not form other complex formers; the smaller ions such as lithium and sodium form some weak unstable complexes with some multidentate agents of high charge such as ethylenediamine-tetraacetic acid and bis-(aminoethyl)-glycol-ether-N,N,N',N'-tetraacetic acid. Therefore, if the salts of patients' urine are composed of mainly univalent small and large alkali metal ions, the urine test kits using this reagent might not get an accurate result for the specific gravity or ionic strength being measured in the test sample. Consequently, there is a need for a method to estimate the ionic strength or the specific gravity of an aqueous environment where most of the metals can be stably complexed, and whose method provides the desired test results in which the color developed by the pH chromogen is maintained constant during the test for a comparatively long period of time.
The present invention departs from the prior state of the art for determining the urine ionic strength or specific gravity in providing a sound improvement of the reagent composition being adopted.